A mobile node may have multiple interfaces like wireless local area network (WLAN), Universal Mobile Telecommunications System (UMTS), General packet radio service (GPRS), Worldwide Interoperability for Microwave Access (WiMAX) and others, and may switch its current communication to any network interface according to user request. An example reason for switching may be that the current wireless network has low signal strength or no signal coverage. However, while switching from one network interface to another, the mobile node may experience difficulties in maintaining the network connectivity and also handoff delays. Handoff delay is measured as the delay from the last to first packet that receiver receives from the sender with original and new network interfaces. For instance, handoff delay may be about 2 to 5 seconds, which may not be acceptable for many real-time services like Voice over Internet Protocol (VoIP) and streaming services. Also, some applications are sensitive to handoff delays (e.g., VoIP services) and some may break due to long delays (e.g., Transmission Control Protocol (TCP) connection).
Existing methods attempt to address some of the above mentioned mobility management problems. Those known methods could be classified according to which protocol layer a method could be applied to: link layer, network layer, or session/application layer solutions. Link layer solutions, however, cannot handle mobility requirements at higher protocol layers, such as change of Internet Protocol (IP) addresses as a mobile node moves from one IP subnetwork to another. Network layer solutions are more general to all applications. A typical network-layer solution is mobile IP. Session layer or application layer solutions are limited to a specific application. Representative examples of session/application layer mobility methods are methods based on the Session Initiation Protocol (SIP).
Mobile IP employs a home agent in the home network of mobile nodes, and each mobile node has a permanent home IP address. When the mobile node roams outside of the home network, it would acquire a care-of address and registers the address to its home agent. All traffic from a corresponding node destined to the mobile node will have its home IP address as the destination address. If the mobile node is currently inside its home network, it could easily get the packets. Otherwise, its home agent will intercept the packets for it and tunnel the packets to its care-of address. Similarly, all traffic sent from the mobile node will have its home address as the source address in the IP header, and the packet will then be tunnelled with its care-of address to the home agent. Upon receiving the packet from the mobile node, the home agent will decapsulate the packet and forward it to the corresponding node.
In mobile IP, all traffic sent from/to mobile nodes need to be tunneled to the home agent. This may cause triangular routing problem when the mobile node is close to the corresponding node and far away from its home agent. Route optimization is used to get around this problem. When the home agent finds there is a corresponding host that wants to communicate with a mobile node away from its home network, the home agent sends a binding update message to inform the corresponding node about the care-of address of the mobile node. The corresponding node could then tunnel the packets to the care-of address directly. If the corresponding node does not know the movement of the mobile node and keeps sending traffic to the old care-of address, the foreign agent will send a binding warning message to the home agent. Upon receiving binding warning message, the home agent sends binding update message to the corresponding node with the update care-of address of the mobile node.
Mobile IP without route optimization needs to build a tunnel from the home agent to the mobile node, so all traffic destined to the mobile node could be forwarded to the mobile node via this tunnel. Unfortunately, unidirectional tunnel may cause ingress filtering problem. The ingress filtering problem happens when routers at foreign networks do not allow packets having source IP address outside their subnets to be sent. Since a mobile node always uses its home address as the source address of all packets, these packets will be filtered by routers in foreign networks. Therefore, in practicality, most implementations of mobile IP establish a bi-directional tunnel between the mobile node and its home agent. All traffic from/to the mobile node will go through this tunnel, and cause inefficiency.
If route optimization is applied, explicit signal messages are still required to inform the corresponding node about the new care-of address of the mobile node. Besides, foreign agent is required at each foreign network to send binding warning messages or the home agent needs to know the list of corresponding nodes of each mobile node so that it could send binding update message to update their information about the new care-of address of the mobile node.
Whether the route optimization is applied or not, mobile IP still has a problem of long handoff delay. Each mobile node needs to register its new care-of address with its home agent before other hosts know the mobile's new care-of address and traffic to the mobile can be directed to this new address. This could lead to heavy delays when the home agent is far away.
In the mobile IP like solutions, the mobile node needs to send registration or binding update message to the home agent or the corresponding node. In such solutions, it is found that at least one round trip time delay from the mobile node to the anchor node is required.
SIP mobility, on the other hand, is a solution designed for SIP sessions. A SIP proxy is deployed on the Internet. When a mobile node changes its IP address, it needs to send a SIP registration message to the SIP proxy to update its current location. Each SIP session is established by SIP proxy. Specifically, a corresponding node would send a SIP invite message to the proxy to request for setting up a session to the mobile node. This invite message will then be forwarded to the current address of the mobile node. If the mobile node moves again during this session, it needs to send another invite message to the proxy to inform the corresponding node that it has changes to another IP address. Afterwards, the session will be established to the new IP address of the mobile node.
SIP, however, has the same drawback of the handoff delay as mobile IP has. A mobile node still needs to send IP messages to the corresponding node or a SIP server to update the mobile's new address. The session will be delayed for at least one round trip time. Besides, SIP-based mobility support solutions typically apply to only SIP services. Not all applications could employ this solution to handle mobility issues.
SIP or other application solutions are also designed for dedicated services. They still need to send update message to the corresponding node or any other elements in the Internet like SIP proxy. Therefore, they could not eliminate handoff delay and could not be applied to all applications.